1. Field of the Invention
The invention relates to a doppler navigation antenna with an automatic land-sea error correction by the production of two somewhat differently inclined lobe groups (beam lobing) consisting each of four lobes, and employing a plane radiator group, consisting of individual radiators arranged in parallel rows, the ends of which are fed by feed lines extending transversely to the rows and provided at both of their two ends, each with feed points.
2. Description of the Prior Art
In doppler navigation methods radar beams are directed from the object in flight toward the ground and the doppler displacement of the reflected waves is measured. Only those speed components cause the known frequency shifts which reflect in the direction of the radar beam. In order to obtain the speed components for the movement in space, at least three measurements have to be made in different directions, which must not lie in one plane. In methods with continuous measurement, this signifies the necessity of providing at least three beams. A fourth beam is usually provided for substitution purposes.
The known methods make possible a relatively high accuracy of measurement in the navigation over land, while over water, above all, an error appears which is due to the dependency of the reflection energy from the incident-beam angle on the water. This error may be explained by realizing that the energy of the ground echo over land is independent of the incident-beam angle, while the return-beam energy over water changes appreciably--both with the incident-beam angle as well as also with the surface condition (motion of the sea). In this manner, the frequency spectrum of the echo signals is distorted with respect to the frequency spectrum received over land, which leads to an erroneous shifting or displacement of the measured frequency center. It is therefore necessary that a correction be made when flying over water.
With most of the systems heretofore employed, this correction is made by employing a so-called land-sea switch. This method, however, as a compromise eliminates solely an error transmitted over water, independently of the motion of the sea, and this results in an additional load on the pilots. Additionally, this method requires a ground-view or the knowledge of the territory flown over.
For the determination of the dependency of the reflected energy from the angle of incidence, which is different for land and sea, an automatic land-sea error correction may be derived. For this purpose, it is known to radiate with a doppler navigation antenna three or four differently inclined lobes to the ground and in addition usually offset in time, and in addition to radiate three of four lobes more which, however, are changed slightly with respect to the first four lobes in their inclination (beam lobing). The difference in energy of two adjacent beams is different and depends on the motion of the sea, so that also a correction dependent on motion of the sea may be derived.
The previously known systems employ for the production of the two groups of lobes, either two separate antennae which are arranged adjacent one another, or two interstacked antennae. Since for the two antennae together mostly only one predetermined surface is available, while with two separate antennae both are only half as large and therewith the beam width in one plane twice as large, and accordingly is less favorable. With two antennae interstacked or telescoped, there are disposed between two antennae, for example, slotted hollow conductors of the one antenna and of the other antenna. In this manner, the distances of the emitters of the one antenna become so large that several main lobes occur. If, however, at one antenna inlet, two main lobes occur, then the signals of these two lobes differ solely through their different doppler frequencies, which can no longer be separated in slowly flying aircraft, for example helicopters.